1. Field of the Invention
This invention relates to a method of making heteropoly acid catalyst compositions which can be used in a process for the oxidation of unsaturated aldehydes, such as methacrolein, to unsaturated carboxylic acids, such as methacrylic acid, in a vapor phase reaction.
2. Description of the Prior Art
Heteropoly acid compounds have a central metal atom surrounded by a framework of other metal atoms connected to each other and the central metal atom through oxygen atoms, i.e., metal oxide clusters forming heteropolyoxoanions in acid form. The central metal atom is different (“hetero”) from the framework metal atoms. Heteropoly acid compounds can contain oxides of metals such as molybdenum, phosphorus, vanadium, cesium, copper, bismuth, boron, antimony, tungsten, cerium, niobium, indium, iron, chromium, cobalt, nickel, manganese, arsenic, silver, zinc, lead, tin, titanium, aluminum, silicon, tantalum, germanium, gallium, zirconium, magnesium, barium and lanthanum. Heteropoly acid compounds are known as catalysts for the gas phase catalytic oxidation of unsaturated aldehydes to unsaturated carboxylic acids.
U.S. Pat. No. 5,618,974 discloses production of methacrylic acid by the vapor-phase oxidation and/or oxidative dehydrogenation of methacrolein, isobutyl aldehyde or isobutyric acid with a catalyst of a heteropoly acid component of a composite oxide of molybdenum and phosphorus and a solid acid component having acid strength of not more than −11.93. Example 23 discloses a solution of ammonium molybdate in water to which phosphoric acid, a solution of cesium nitrate in water, bismuth nitrate and antimony pentoxide in powdery form and a solution of chromic anhydride and selenium dioxide in water were added.
U.S. Pat. No. 5,550,095 discloses a process for synthesis of methacrylic acid from methacrolein with a solid catalyst obtained from extrusion molding a mixture of methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol or acetone and the dried heteropoly acid product from an aqueous slurry containing at least molybdenum, phosphorus and vanadium. Example 5 discloses a homogeneous bismuth nitrate solution obtained from heating nitric acid, water and bismuth nitrate added to a solution of ammonium paramolybdate, ammonium metavanadate, potassium nitrate, cesium nitrate, phosphoric acid, arsenic acid, cerium nitrate and copper nitrate. Antimony trioxide was added after the bismuth nitrate solution.
U.S. Pat. No. 5,532,199 discloses a carrier-supported catalyst for the synthesis of unsaturated aldehydes and unsaturated carboxylic acids. The catalyst has at least molybdenum and bismuth as components of a composite oxide and glass fiber as a carrier. Example 1 discloses a solution of nitric acid, water and bismuth to which ferric nitrate, nickel nitrate, cobalt nitrate, magnesium nitrate, zinc nitrate and boric acid were adder to form a solution which was added to a solution of ammonium paramolybdate, ammonium paratungstate, cesium nitrate and antimony trioxide in water.
U.S. Pat. No. 5,206,431 discloses a process for producing methacrylic acid with a plurality of heteropoly catalysts of different activities in a plurality of reaction zones. Example 13, discloses a solution of ammonium paramolybdate and ammonium metavanadate in heated water to which orthophosphoric acid, an aqueous solution of strontium nitrate, and palladium nitrate, powdery bismuth nitrate, powdery ammonium tungstate and an aqueous solution of chromic anhydride and selenium dioxide were added.